The reproductive ecology of the Pepperbark tree (Warburgia salutaris) in the Kruger National Park, South Africa

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2021

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Van den Bosch, Kaylee

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Warburgia salutaris, the pepperbark tree, is an endangered tree that is greatly valued in South Africa for its use in traditional medicine. Over-exploitation for medicinal purposes, habitat degradation and loss have all contributed to the species being listed as IUCN Endangered. In the Kruger National Park (KNP), W. salutaris fruit production is low and only produced on a few trees and there is a very low proportion of juveniles, suggesting that recruitment is extremely limited in this population. Therefore, in this thesis, the reproductive ecology of the KNP population is studied and compared with a reproductively successful orchard population in Eston (KwaZulu-Natal), to investigate factors that may contribute to limited sexual output in the former. The aim of the first part of the study was to understand the pollination biology of W. salutaris. Potential pollinators were observed and collected for further analysis; nectar volume and sugar concentration were measured; five pollination treatments – autonomous self-, geitonogamous self-, cross-, natural- and natural-bagged pollination (i.e. naturally pollinated flowers were bagged to protect from seed predation) – were set-up to elucidate the breeding system; pollen viability was assessed, and pollen tube growth was compared between pollination treatments. The aim of the second part of the study was to understand the seed ecology of W. salutaris. Immature fruit were bagged to assess fruit development in the KNP population and Eston orchard; fruit were analysed to assess the effect of pre-dispersal seed predation and identify seed predators; frugivore activity was observed in Leshiba Game and Nature Reserve (GNR), Limpopo, to identify seed dispersers and assess the mode of dispersal; and seedling emergence trials were conducted to compare seed germination between the natural KNP and Leshiba GNR populations and the Eston orchard. Results showed that W. salutaris flowers are visited by a variety of insects and Lepidoptera (butterflies and moths) are the most likely effective pollinators. Average nectar volume and sucrose concentration was 2.43 l and 39.74%, respectively. Warburgia salutaris is likely self incompatible as self-pollinated flowers in neither KNP nor Eston resulted in fully developed fruit. In KNP, zero mature fruit were produced from either self- or cross-pollination; however, a small percentage of self (5%) and cross (2%) pollinated flowers were fertilised and then aborted early in development. Similarly, 94% of flowers that began to form fruit in the natural-bagged treatment were aborted. Only two trees in one KNP sub-population produced and sustained fully developed fruit through natural pollination in 2019. In 2020, the same two trees and an additional untagged v tree successfully developed fruit. Pollen viability was high (66–93%) across the five KNP sub populations and Eston, and percentage pollen tube occurrence for self- and cross-pollination was higher in Eston (self: 60%, cross: 100%) than KNP (self: 33%, cross: 33%), and both self and cross pollen tubes reached the ovules. Bagged immature fruit were aborted early in development in the KNP population and Eston orchard, and in KNP successfully developed fruit were only present on three of 10 trees in one sub-population. Ceratitis cosyra, the marula fly, attacks W. salutaris fruit in KNP, Eston, and Leshiba GNR. However, 80% of collected KNP fruit were unaffected by seed predation. Seeds from mature fruit with immature larvae had a far higher germination percentage (77%) than seeds in fruit with fully developed larvae (6%). Therefore, there is a short period where seeds remain viable while infested with C. cosyra. Frugivore observations in Leshiba GNR revealed that baboons and vervet monkeys eat W. salutaris fruit (except the exocarp) and ingest the seeds. However, their role as effective dispersers of W. salutaris is not certain. Frugivore activity was not observed in KNP, but evidence of frugivore activity (i.e. empty fruit exocarps) was found. In the seedling emergence trials KNP had the highest percentage germination (61%), followed by Eston (47%), and Leshiba (33%), and all ungerminated seeds were nonviable. This study has expanded our knowledge of the reproductive ecology of W. salutaris and confirms that sexual reproduction of the species in KNP results in very low fruit and seed production. However, W. salutaris has many floral visitors, pollen viability is high, and seeds that are produced germinate successfully. Therefore, the impaired reproductive output in KNP appears to be in the high occurrence of aborted fruit, which was found for both self- and cross hand-pollinations and natural-bagged pollination treatments. For self-pollination this may be late-acting self-incompatibility, i.e. the ovule is aborted after it has been fertilised by self-pollen. However, the even lower success of cross-pollination may be explained by a genetically linked post-fertilisation failure. This is further supported by the higher percentage of pollen tube occurrence compared to fruit production for both self and cross hand-pollinations. The already limited fruit production in KNP is impacted further by pre-dispersal seed predation, lowering the number of seeds that can eventually contribute to seedling recruitment. However, germination of the desiccation-sensitive seeds and seedling establishment may only be possible in seasons with fairly regular and high rainfall. Therefore, it is possible that W. salutaris relies on clonal reproduction and resprouting in response to damage to persist in northern KNP. However, long term reliance on clonal reproduction may impair the ability to reproduce sexually, which will ultimately be necessary to establish genetically diverse genets that have the potential to adapt to future environmental changes.

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A dissertation submitted in fulfilment of the requirements for the degree Master of Science to the Faculty of Science, School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Johannesburg, 2021

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